Electroplated corrosion proof metal articles and method of making the same



Patented Apr. 22, 1947 ,ELEC'I'ROPLATED CORROSION PROOF METAL ARTICLESAND METHOD OF v MAKING THE SAME Donald H. Schantz, Coraopolis, Pa.,assignor Standard Steel Spring Company, Coraopolis, Pa., a corporationof Pennsylvania No Drawing. Application December 21, 1940, Serial No.371,100

9 Claims. ((129-1915) of metallic coatings to'the surfaces of thearticles which are to be protected. Its main object is to provideeffective coatings having many advantages over any prior coatings knownto the applicant. v

The protection afforded by applied metallic coatings arises mainly fromthe degree to which they are continuous or free from defects. Untilrecently, at least, .it has been generally assumed by workers in the artthat the protective value of metallic coatings also depends upon theposition of the coating materials in the electro-chemical scale relativeto the base metal. Lately, doubts have arisen whether that theory is asound one, but it is used in this specification only as a convenientbasis for explaining the invention. Nickel and copper, for example, are,

according to the theory, electro-negative to iron may actuallyaccelerate corrosion of the underlying metal.

Zinc is electro-positive to iron and a zinc coating on ironor steel willgive substantially complete protection if the coating is free frombreaks or defects exposing the underlying metal. If there are breaks ordefects in the zinc coating the underlying or base metal will notcorrode Or rust so long as there is zinc available on the surface of thearticle in the vicinity of a break or defect. It is difficult, if notimpossible, to get a zinc coating in comrr'ierically practicalthicknesses having perfect continuity, and, in addition, such coatingsare subject to accidental damage. The electrolytic potential of zincrelative to iron is such that the consumption or dissolution of zincwhen it starts to give way under corrosion is more rapid than isnecessary for good protection, so that the effective life of the coatingis shorter than it should be.

An object of the invention is to provide a method of applying aprotective coating, which is singularly free from pores, pinholes anddefects of a similar nature, and which is also of such a compositionthat it is not easily damaged.

Another object is to employ zinc for protective coating purposes, but insuch a way that the difference in electrolytic potential between thecoating and the metal base is reduced to a point intermediate that ofzinc and iron. This result is accomplished by coating th surface of thearticle with an alloy of nickel and zinc which is less anodic to iron orsteel than zinc, thereby slowing down the decomposition of the coatingin the case of a break or defect through which harmful agents mightpenetrate to set up electrolytic action. 1

Still another objectis to make it possible to electroplate alloys ofnickel and zinc on a commercial basis. This has not previously been donesuccessfully. This is done by employing a new buffered electrolyte, highin metal content, and using quite high'current densities. The preferredrange of density is from 300 to 700 amperes per square foot of cathodearea, although densities below and above the ones specifically mentionedmay be used. The invention further includes new correlations oftemperature, pH or hydrogen ion concentration, and buffer concentration,which may be varied to control the percentages of nickel and zinc in thealloy deposited. Microscopic examination of a typica'Fnickelzinc alloy,formed by heating the two metals while in contact, shows that the alloyhas a 25 stratifled appearance and analysis shows that the nickel andzinc, in what are for convenience termed the strata or layers of alloy,conforms in structure to the nickel-zinc alloy system as follows:

Alpha Up to 28%, Zn Beta (sub-one) -44% Ni Gamma prime 24-20% Ni Gamma20-13% Ni Delta. 12-10% Ni gamma alloy is immersed in acorrodingsolution its potential with respect to the solution is onlyslightly less electro-negative than that of zinc immersed in the samesolution. when, however,

gamma is coupled to iron and the potential of 50 a the couple ismeasured, the resulting value is the same as that'obtained' for ironalone; when zinc is coupled to iron, the potential of the combination isthat of zinc. It may b statedfrom this that the gamma-iron couple isunder "anode con- 58 tro "-1, e., the area of the anode (gamma)consuccessively stripping ofi the difi'erent strata or trols the rate ofcorrosion. For the zinc-iron combination, the couple is under "cathodecontrol"- i. e., the area of the cathode (steel) controls the corrosionrate. Thus for zinc coated steel, as any pore in the zinc coat isenlarged by corrosion, the area of the cathode increases considerably,and the corrosion rate correspondingly increases. As

. a pore in gamma-coated steel is enlarged, the anode area is notchanged much, and therefore the corrosion rate remains constant. Thismay in part be due to the physical structure of the gamma alloy, or itmay be due in part to the fact that the gamma alloy has greatercontinuity, or it may be almost entirely due to the fact that the gammaalloy is only slightly electro-positive to iron or steel. While, asstated, the gamma alloy seems to be the best from the standpoint ofprotection, the gamma prime and delta a1- loys are effective, but to alesser extent than the gamma alloy. In fact, nickel-zinc alloys havingan amount of nickel less' than the amount found in th delta alloy giveworthwhile results. Apparently, however, pronounced benefit from thenickel-zinc alloys is found only when the percentage of nickel issomewhere between about 3 percent and about 24 percent, with about 11 Ito 18 percent giving best results.

sulphate calculated as ZnSO4.7HzO or about 16.7

ounces of zinc chloride ZnClz. The zinc metal content should be fromabout seven to about nine ounces. Nickel is introduced as a chloride.About eight ounces of nickel metal should be introduced by adding to theelectrolyte about thirty-two ounces of NiCI2.6H2O. The electrolyte alsocontains a buffer which may be acetic acid, formic acid or citric acidat a concentration of about .5 normal. The salts of some of these acidsmay be used. This buffered electrolyte may have,

in operation, a pH of from about 1 to about 3, but a pH of from 1.5 to2.5 is preferred. The pH may be adjusted by using hydrochloric acid tolower it, and either nickel or Zinc carbonate or zinc oxide to raise it.The electrolyte should be kept in circulation in a well known mannerwhen the articles being plated are simply suspended in the electrolyte,but in plating continuously upon wire or strip moving through theelectrolyte such circulation may not be necessary.

It is preferred to use separate nickel and zinc anodes having theirsurface areas relatively proportioned on the same ratio as the nickeland zinc in the, desired alloy. In order to get the right distributionof the current to the cathode, the article which is receiving thedeposit, it is preferred to use a plurality of zinc anodes and aplurality of nickel anodes and so position them on the anode support asto get the current distribution desired.

Instead of using separate zinc and nickel anodes, cast or rolled alloyanodes having the metals in about the same proportions as in the alloyto replace the zinc plated out; or zinc anodes may be used and thenickel replenished by adding nickel salts. Insoluble anodes may also beused, but this requires constantly adding both the zinc and the nickelsalts to replace the metals which have been plated out.

The operating temperature for an electrolyte made up as above directedmay be from 70 to 150 F., a temperature of 125 F. giving good results inplating an alloy containing 11 percent to 18 percent nickel.

The current generators employed are of the usual type, the voltagesrunning from 6 to 24 volts, depending both flipon the distance from theanode to the cathode and'the amperage desired. For coating sheets andstrip about 300 amperes to the square foot of cathode surface appears tobe best, regardless of whether the material is suspended in theelectrolyte or is fed continuously through the electrolyte. Wire can beplated at about the same amperage per square foot as is mentioned forsheets and strip, but better results commercially are obtained in thecontinuous plating of wire if the current density is 500 amperesorhigher per square foot of cathode surface. It is important in allinstances that the current density selected fOr a particular alloyplating operation shall be kept uniform,

The electrolyte constituted as above specified, and the preferredarrangement of zinc anodes and nickel anodes receiving a currentadjusted to about 300 amperes per square foot of cathode surface, willsimultaneously deposit on the cathode a nickel-zinc alloy in which thezinc and nickel will be present in percentages averaging approximately85 percent zinc and 15 percent nickel.

The percentage of-zinc in the alloy may be controlled by raising andlowering the pH of the electrolyte, by raising or lowering the amperagewithin definite limits, by changing the temperature of the electrolyte,by the use of zinc sulphate instead of zinc chloride when the lowerpercentages of nickel are desired, by regulating the degree of agitationor rate of circulation of the electrolyte, and by changes in therelative amounts of zinc and nickel in the electrolyte. Such changes area matter of experience with particular products. The total metal contentto the gallon of electrolyte should, however, generally be at leastfourteen ounces for the best results with high current densities.

The following are some examples illustrating changes in the procedure toget a desired percentage of nickel and zinc in the alloy.

For continuous plating of the alloy on a wire, a solution has beenemployed which contained ,8 counces of nickel metal (about 35 ouncesNiClaBI-IzO), 10 ounces of zinc metal (about 21. ounces ZnClz), andacetic acid 3 percent by volume. 2.2 and the temperature of the solutionwas raised to 124 F. with the wire traveling at seven and one-half feetper minute through the solution, a current of 515 amperes per squarefoot of cathode area deposited on the wire a coating of alloy containing14 percent nickel and the remainder zinc. Sheets suspended in anelectrolyte in which the nickel metal was 7.2 ounces to the gallon andthe zinc 8.5 ounces to the gallon and which had a temperature of 115 F.and a pH of 2.3 deposited 10 percent'nickel and percent zinc when acurrent density of amperes per square foot of cathode surfacewasemployed. The zinc was introduced is sulphate form.

The pH of the solution was adjusted at form) to the gallon with a pH of2.4 deposited aeiaasi 1. The method of plating articles with anickelzinc alloy which includes making the articles cathode in anelectrolyte, having a pH between about 1 and about 3 in which nickelchloride and zinc salt of the group consisting of zinc sulphate What isclaimed is:

. and zinc chloride have been dissolved in sufilcient 20 percent nickelwhen the current density of i 400 amperes per square foot cathodesurface was employed and the temperature of the electrolyte raised to145 F.

In all of the examples just mentioned the nickel was introduced as achloride and the bufier was acetic acid.

The alloy may be plated directly upon the metal article after thesurface thereof has been properly cleaned, and the alloy coating sodeposited is adherent, continuous and protective to pronunced degrees.It is preferred, however, to first deposit upon the surface of thearticle a primary coating of copper or nickel (nickel is preferred) from0.000025 to 0.000100" thick. Nickel is in itself a protection againstcorrosion if continuous and,

'in addition, being electro-negative to steel, it

probably at least slows down the electrolytic action between the anodicalloy and the base metal where the latter is exposed.

The nickel plated article is next rinsed and then given a coat of alloyplating from the nickelzinc electrolyte. The thickness of the alloycoating may vary within a wide range, being dependent upon theenvironment in which the article is to be used. Usually a thickness of0.000300" is ample for protection against all ex cept very severeconditions.

Also in the foregoing, reference has been made according to thisinvention have shown no signs oi'corrosion of the base metal afterhaving been subjected to the standard salt spray tests for hundreds ofhours. The process is a simple one to follow and the resulting productis better from a corrosion-resistant standpoint then anything found inthe prior art, with many other advantages present which are not found inproducts resulting from any prior practice. The equipment necessary is,of course, more or less standard in the plating art.

In explaining the invention, various examples have been given, but it isapparent that they were illustrative only. It is obvious that theinvention can be employed in other ways to produce various products, allcoming within the claims which follow.

I formic acid, citric acid, and salts thereof, the

amounts to give, to each gallon of the electrolyte,

a metal zinc content of about 7 to 9 ounces and a nickel metal contentof about 7 to 8 ounces, and passing a current having a density of to 700amperes to the square foot of cathode surface through the electrolyte tothe articles.

2. The method of plating articles with a nickel-'- zinc. alloy whichincludes making the articles cathode in an electrolyte consisting of, tothe gallon of water, about 17 ounces of zinc chloride, about 32 ouncesof nickel chloride and about 3 percent by weight of a buffer selectedfrom the group consisting of acetic, citric and formic acid and passingthrough the electrolyte to the oathodes a current of from about 100 toabout 700 amperes per square foot of cathode area, the amperage and thetime during which the article is in the electrolyte depending upon thepercentage of nickel and zinc and thickness of deposit desired.

3. A ferrous metal article having an adherent protective coatingconsisting of a layer of electrodeposited nickel, and an outermostexposed alloy layer at least as thick as the nickel layer consisting ofnickel and zinc simultaneously deposited on the nickel layer from asingle electrolyte and containing an average of about 15 percent nickel.

4. A ferrous metal article having'an adherent protective coatingconsisting of a layer of electrodeposited nickel, and an outermostexposed layer at least as thick as the nickel layer consisting of nickeland zinc-simultaneously deposited from a single electrolyte andcontaining from about 10 percent to about 24 percent nickel and the rest2111C.

5. A plating solution having a pH of over about 1.0 and under about 3.0containing, to the gallon of solution, about thirty-five ounces of zincsulphate, about thirty-two ounces of nickel chloride, and about 3percent by volume of a buffer of the group consisting of acetic acid,formic'acid, citric acid, and salts thereof, the rest of the solutionbeing water.

6. A plating solution having a pH between about 1.0 and about 3.0containing, to the gallon; about l6.7 ounces of zinc chloride, aboutthirtytwo ounces of nickel chloride, and about 3 percent by volume of abuffer of the group consisting of acetic acid, formic acid, citric acid,and salts thereof, the rest of the solution being water.

7. A plating solution containing to the gallon of solution, aboutthirty-five ounces of zinc sulphate, about thirty-two ounces of nickelchloride,

about 3 percent by volume of a buffer of the group consisting of aceticacid, formic acid, citric acid, and salts thereof, and addition agentsadjusting the pH to about 2.5, the rest of the solution being water.

8. A plating solution having a pH of over about 1.0 and under about 3.0containing, to the gallon of solution, a salt of the group consisting ofzinc sulphate and zinc chloride in amount to give about '7 to 9 ouncesof zinc, nickel chloride to give about '1 to 9 ounces of nickel, and abufferingagent of the group consisting of acetic acid,

rest of the solution being water.

give about 7 to 9 ounces of zinc metal to the gal- 10 ion, and nickelchloride in amount to give about 7 to 8 ounces of nickel metal to thegallon, and plating nickel and Zinc simultaneously on the cathode bypassing a current of over 100 amperes and under 1,000 ampere to thesquare foot of 15 cathode through the electrolyte to the cathode. DONALDH. SCHANTZ.

v 8 REFERENCES crrEn The following references are of record in the 5file of this patent:

UNITED STATES PATENTS Number Name Date 'Schoch et al 1907 1,564,581 KingDec. 8, 1925 OTHER REFERENCES Transactions of the ElectrochemicalSociety, vol. XI (1907), Schock et al., pp. 136-139. (Copy in Div. 36.)l

Trans. Electrochem. Soc; vol. 73 (1938), pp. 417-433. (Copy in Div. 56.)

